A known electric linear motion actuator for converting the rotary motion of an electric motor to linear motion, thereby linearly driving a driven member, includes a motion converter mechanism such as a slide screw mechanism, a ball-screw mechanism or a ball-ramp mechanism for converting the rotary motion of the rotor shaft of an electric motor or of a rotary member to which the rotation of the rotor shaft is transmitted to the linear motion of an output member for linearly driving the driven member. Many of this type of electric linear motion actuators include a gear type speed reducer such as a planetary gear type speed reducer so that a large linear driving force is obtainable with a small-capacity electric motor (see e.g. JP Patent Publication 6-327190A).
The inventors of the present invention proposed electric linear motion actuators which can generate a large linear driving force without using a separate speed reducer and which is suitable for use in an electric brake system, of which the linear stroke of the movable member is relatively short (see e.g. JP Patent Publication 2007-32717A and JP Patent Publication 2007-37305A). These actuators include a rotary shaft member which is one of the rotor shaft of the electric motor and a rotary shaft to which the rotation of the rotor shaft is transmitted, an outer ring member provided around the rotary shaft member and fitted in the radially inner surface of a housing, a plurality of planetary rollers rotatably supported by a carrier so as to be disposed between the rotary shaft member and the outer ring member, the planetary rollers being configured to revolve around the rotary shaft member while rotating about the axes of the individual planetary rollers when the rotary shaft member rotates, wherein a helical rib is formed on the radially inner surface of the outer ring member and each of the planetary rollers has on its radially outer surface circumferential grooves which are equal in pitch to the helical rib or a helical groove which is equal in pitch to, and different in lead angle from, the helical rib, the helical rib being engaged in the circumferential grooves or the helical grooves of the respective planetary rollers, whereby the rotary motion of the rotary shaft member is converted to the linear motion of the carrier as the output member for linearly driving the driven member, thereby linearly moving the outer ring member and the carrier relative to each other.
Most brake systems for vehicles had been of hydraulic types. But due to recent introduction of sophisticated brake control arrangements such as antilock brake systems (ABS), electric brake systems, which can perform such brake control without the need for a complicated hydraulic circuit, are gathering much attention. Electric brake systems include an electric motor and an electric linear motion actuator of the above-described type, which is mounted in the caliper body of the brake system. When the brake pedal is depressed, a signal is produced to activate the electric motor, and the motor drives the braking member through the actuator, thereby pressing the braking member against the braked member (see e.g. JP Patent Publication 2003-343620A).
This type of electric brake systems, which include the above-described electric linear motion actuator, have means for controlling the braking force. One of such means calculates the motor torque based on the current being supplied to the electric motor and adjusts the current to be supplied to the motor to a predetermined value corresponding to the required braking force. Another such means detects the angular position of the electric motor and adjusts the angular position of the motor to a predetermined position corresponding to the required braking force.
With the arrangements in which the required braking force is produced by applying the predetermined current to the motor or rotating the motor to the predetermined angular position, it is difficult to accurately linearly move the output member to the required position due e.g. to manufacturing errors of component parts forming the electric linear motion actuator or slip of the motion converter mechanism, which in turn makes it difficult to produce the required braking force.